The objective of this proposal is to establish the feasibility of a broadly applicable technology to measure gene expression in single cells, without the need to perform technically challenging manipulations on individual cells. Currently, large-scale genetic screens based on new technologies are leading to important advances in our understanding of mammalian genomes and genetic variation linked to human disease. However, the power of these screens is limited by the lack of methods to measure gene expression in these large screens. Our proposed technology would fill this major unmet need, and thus have a broad impact on mammalian genetics. Our goal is to develop a method to attach single-cell-specific sequence barcodes to transcripts, using RNA proximity ligation in pooled samples. We propose to do this by designing barcoded ?tagRNAs? which are expressed in cells, and targeted to specific RNA binding proteins. These tagRNAs are attached to transcripts derived from the same single cell by proximity ligation. To establish the feasibility of this method, our aims are 1) to robustly express tagRNAs that are stably attached to their targets, and 2) to test the single-cell specificity and efficiency of RNA proximity ligation to barcode transcriptomes.